Embodiments of the invention relate generally to a method of calibrating an MRI system and more specifically for calibrating an RF coil when positioned within the MRI system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR systems typically include radio-frequency (RF) transmit coils that are used to transmit electromagnetic waves into a sample, creating the B1 magnetic field needed to excite nuclear spins. Receive coils detect the signal emitted by the nuclear spins as they precess in the B1 field. The same coil may be used for both exciting spins and receiving the resulting excitation signal, or transmission and reception may be performed by separate coils which are constructed to minimize coupling therebetween.
Various types of coils include for instance solenoidal coils to generate an RF magnetic field (B1 field), surface coils and phased arrays, and volume resonators. In the case of a volume resonator, a variety of types are available that may be defined as cylindrical, multi-loop coils which generate the B1 field perpendicular to the bore axis. A birdcage coil is commonly used as a head or a body coil, and can be used in both transmit-receive and transmit-only configurations. The birdcage coil can be operated in quadrature mode in order to reduce power and also to achieve an increase in B1 field strength and detection sensitivity (in, for instance, a transmit-receive configuration). It is well known that in free space, a quadrature operated volume birdcage coil produces and receives a circularly polarized B1 field. For lossy objects with a relative permittivity similar to water and circular in cross-section and at a frequency at which the wavelength is comparable to the human body, the B1 field may only be truly circularly polarized over a small region at the center. Nevertheless, for a circular cross-sectioned object, a circularly polarized B1 field is considered the most efficient in terms of B1 field generated for a given amount of power.
In transmission mode, the driving current is split into two signals which are applied to the birdcage coil in order to create, theoretically, a circularly polarized field using sinusoidal currents of equal magnitude that are 90° out of phase. That is, one field is driven sinusoidally in-phase (i.e., the I port), and the second field is driven sinusoidally in quadrature (i.e., the Q port). The fields add as vectors in quadrature, with the final B1 field oriented perpendicularly to the bore axis. In reception mode, a birdcage coil simultaneously detects components of B1 along two orthogonal directions, yielding two separate electrical signals. As such, theoretically at least, a circularly polarized B1 field may be generated from operation of the I and Q ports in quadrature and having the same amplitude and being 90° out of phase. However, as will be described, a circularly polarized B1 field may not be achieved due to the presence of a body being imaged and may not be achieved due to variations in components that are used to fabricate the MR system.
During manufacture, coils may have a varying capacitance as a function of angle within the coil for a number of reasons including but not limited, such as if the coil is formed out-of-round, the coil shield is out-of-round, or if the antenna (rung and endring) to shield distance varies as a function of location. This variation may be due to variability of components themselves (i.e., capacitors, cable lengths, etc. . . . ) and to variations in component dimensions during the manufacturing process, as examples. Thus, even though a coil may be fabricated to exacting tolerances and with components having very tight specifications, coil variation nevertheless may occur due to a cumulative effect of all the components of the coil. Such variation manifests itself as a non-uniform coil capacitance as a function of angle within the coil which, at system level and during MR operation, results in a non-uniform and typically elliptical field when driven in quadrature from the I and Q ports.
Methods have been developed in order to correct for the capacitance variation of the coil as a function of angle. For instance, one known method of tuning a birdcage coil (i.e., with a rung and endring) includes measuring the coil and adjusting the capacitance thereof prior to installation into the MR system. In this example, two flux probe are used wherein the first flux probe is used to excite the birdcage coil and the second flux probe is used to measure the B1 field. Using the two flux probes, coil tuning is measured for a number of angular orientation and the coil capacitance may be changed to tune the coil using known methods (i.e. replacing individual capacitors about the coil). For lower B1 field systems, such as 1.5 T or below, such tuning generally proves to be adequate.
Also, in a higher B1 fields, at or above 3T for instance, the wavelength is short and can interact with the wavelength of a body being imaged. As such, the B1 field combines with the wavelength of the body, leading to an inhomogeneous B1 field when the wavelength in the body is comparable to the body being imaged, that can manifest itself as shading in a final image. This is known as the “dielectric effect,” which can be compensated for by driving the coil in an elliptical mode in order to compensate for this effect and reduce the shading. That is, the I and Q ports can be driven having either their phases shifted (different from 90°) out of phase from one another, their magnitudes varied from one another, or both. This results in an elliptical polarization with precise orientation and strength that minimizes the B1 inhomogeneity within the imaged object. Thus, by knowing the effect of the body, phases and magnitudes of ports I and Q can be selectively driven in order to compensate for and minimize the dielectric effect.
As such, known MR systems include coils that can be tuned at the coil level, and known MR systems may also correct for the dielectric effect by driving an elliptical polarization, as discussed. However, one factor that is not accounted for in known systems is the system level interaction of the coil with other components once the B1 field generator, such as a birdcage, is installed into the system. Once the birdcage coil is electrically connected to the rest of the system overall capacitance is affected despite having a coil that has been tuned at the coil level. And, although not negligible the system level effect is sufficiently small for systems having a lower B1 field strength, such as below 1.5 T. As such, known systems having a birdcage coil may successfully operate using a tuned coil and using an elliptically driven polarization to correct for the shading caused by the dielectric effect. Such operation, though, is inherently based on an assumption that connecting the coil to the system did not affect overall capacitance.
As B1 field strength is increased to a field strength such as 3 Tesla, however, the phenomenon known as the dielectric effect becomes more of a problem. That is, for high B1 field strength the problem is exacerbated and shading as a result of the dielectric effect can be more pronounced, resulting in a larger needed polarization ellipticity. And, although known algorithms may include phase and/or magnitude shifts to compensate for the dielectric effect, such compensation is nevertheless based on an assumption that the B1 field is uniform to begin with. Thus, compensation for the dielectric effect may have only limited benefits if the beginning assumption of B1 field uniformity is not correct.
Thus, the effect of system-to-system B1 field non-uniformity may be small at 1.5 T, and an assumption of B1 field uniformity along with elliptical drive compensation for the dielectric effect is typically adequate to avoid shading. However, at higher B1 field strength, such as at 3 T or above, such assumptions can fall apart, leading to shading and a need to reduce system to system variation once the B1 field generator is installed into the system.
It would therefore be desirable to have a system and method capable of compensating for unit-to-unit variation and tuning a B1 field of a coil for a system driven in quadrature.